Alloy steels



United States Patent 3,472,707 ALLOY STEELS Rodney Phillips, Totley, Shelfield, England, assignor to The British Iron and Steel Research Association 7 No Drawing. Continuation-impart of application Ser. No.

448,244, Apr. 15, 1965. This application Jan. 19, 1968, v

Ser. No. 699,039

Int. Cl. C22c 39/50, 39/30, 39/54 US. Cl. 14836 Claims ABSTRACT OF THE DISCLOSURE This invention is concerned with improvements in or relating to alloy steels with ferritic microstructures.

This application is a continuation-in-part of my application Ser. No. 448,244 filed Apr. 15, 1965, now abandoned.

One of the greatest problems facing the steel industry since the introduction of mild steel as a commercial structural material in 1862 has been to discover how to increase the strength of this steel without at the same time incurring too great a loss of toughness and weldability. Relatively little progress has been made, as can be seen by the fact that the allowable working stress for the most common structural steel (i.e., that meeting British standard specification BS. 15, 1961, which calls for a maximum carbon content of 0.25% and a minimum yield stress and a minimum ultimate tensile strength of 16 and 28 tons per square inch respectively) has been raised from 4 /2 tons/sq.in. to only 9 /2 tons/sq.in. in 100 years. In a much shorter time, that is in the period since 1920, the allowable working stress of reinforced concrete, the principal competitor to structural steel, has increased by 500%.

Many attempts have been made to improve the competitive position of structural steel by increasing the yield strength. One technique has been to raise the carbon content and introduce alloying elements, such as manganese, to improve toughness and aid grain refinement. A typical example of such a practice is steel meeting British standard specification BS. 548, 1934 in which a yield stress and an ultimate tensile strength of 23 tons per square inch and 37 tons per square inch (as compared to the 16 tons per square inch and 28 tons per square inch of British standard specification BS. 15, 1961) is achieved by raising the maximum carbon content to 0.30% and the maximum manganese content to 1.5%.

Welding difficulties were experienced with this type of steel due to the high carbon content, and a lower carbon, higher manganese British standard specification BS. 968, 1941 was introduced. This specification calls for a maximum carbon content of 0.23%, a maximum manganese content of 1.8% and the same minimum yield stress and ultimate tensile strength as BS. 548, 1934. Difficulties with welding this steel and in controlling its toughness have also been experienced, the most notorious example being the failure of the King Street Bridge in Melbourne, Australia in 1962.

A revised BS. 968 specification (British standard specification BS. 968, 1962, which specifies a maximum carbon content of 0.20%, a maximum manganese content of 1.5%, a minimum yield stress of 23 tons per square inch, a minimum ultimate tensile strength of 32 tons per square inch, and a Charpy V notch impact energy of 20 foot- 3,472,707 Patented Oct. 14, 1969 "Ice pounds at 15 C.) was then introduced in which both the carbon and manganese were reduced but advantage was taken of the grain refining effect of the element niobium (see US. Patent 2,158,651) to retain the minimum yield strength of 22 tons/sq.in. Even this steel is, however, not free from welding and toughness problems.

All the above steels are of the type known as ferrite/ pearlite steels in which there are two microstructural constituents, ferrite, which is almost pure iron, and pearlite, which is a lamellar mixture of ferrite and iron carbide. The presence of the pearlite has always been considered essential because of its strengthening efliect and because its precipitation in the ferrite restrains ferrite grain growth and thus adds to yield strength and toughness.

I have now found that, because other strengthening methods are now available and because strong carbide formers, such as niobium, exert a strong grain refining effect, pearlite is no longer essential as a constituent of high strength structural steels. I have further found that if the carbon content and the other alloying constituents of the steel are so chosen that the amount of pearlite present in the structure is less than 2% by volume, the resulting steel is remarkably tough and is also readily weldable.

According to the present invention, I provide an alloy steel having a ferritic microstructure containing precipitated carbides or carbonitrides of at least one of the elements niobium and vanadium and not more than 2% by volume of pearlite, a yield strength minimum of 22 tons per square inch, a Charpy impact transition temperature at 40 ft. lbs. of less than 0 C. and a Charpy shelf impact absorbed energy value of more than ft. lbs., said alloy steel consisting essentially of up to 0.08% carbon, from 0.85 to 2.5 manganese, from 0.001 to 0.03% nitrogen, up to 0.5% silicon, and at least one of the elements niobium and vanadium in the proportions 0.01 to 0.20% Nb and 0.01 to 0.30% V, the valance being iron and incidental impurities and quantities of such elements which may be required to facilitate the steelmaking practice.

The preferred manganese content is from 1.25 to 2.00%, but if extra high yield strength is required then up to 2.5% can be tolerated or if the best possible weldability is required, the manganese content can be reduced to 0.85%.

While 0.08% is the stated upper limit of carbon content, the steel may, according to a modification of the invention, contain up to 0.10% carbon. When a carbon content between 0.08 and 0.10% is used, the niobium and vanadium content should be at the upper ends of the ranges mentioned above, that is at least 0.06% in the case of each of these carbide-forming elements, in order that the steel should not contain more than 2% by volume of pearlite. The steel may also contain up to 1.2% silicon, but 0.5 or less silicon is preferred.

For steels containing 0.08% or less of carbon, the preferred content of each of the carbide-forming elements is from 0.03 to 0.06%.

The incidental constituents which may be present in the steels according to the invention are, firstly, the impurities which normally arise in steelmaking practice, such as sulphur (typically up to 0.025%) and phosphorus (typically up to 0.05%), and secondly, small quantities of additional alloying elements, such as nickel (suitably up to 0.5%) where additional toughness is required, and chromium (suitably up to 0.5%) or copper (suitably up to 0.5%) where additional corrosion resistance is required.

The alloy compositions according to the invention are suitable for production as rimming or semi-skilled steel.

The steels according to the invention can be normalised or rolled at low or high finishing temperatures according to the properties required in the finished steel. Thus products with particularly good yield stress properties are obtained by using the steels in the rolled condition, the rolling schedule being arranged so that the final pass is made in the temperature range 650 C. to 950 C.

Toughness in steels is measured in several ways but the two clearest indications of the toughness of a structural steel are the Charpy impact transition temperature and the Charpy shelf impact absorbed energy. The lower the first and the higher the second, the tougher the steel. Reducing the pearlite content from its normal value of about 15% to 2% can reduce the transition temperature by 30 C. Reducing the pearlie content to can reduce the transition temperature by a further 60 C. The Charpy shelf impact absorbed energy can be raised simultaneously from a normal value of 80 ft. lb. to one of more than 220 ft. lb.

Any loss in strength occasioned by the removal of the pearlite to these low limits is more than compensated for in the steels according to the invention by the presence of the niobium and vanadium which contribute strength by precipitation hardening and grain refinement, by using a manganese content at the upper end of the above-mentioned range, if necessary, and by finish rolling at low temperature to improve grain refinement.

The removal of pearlite so improves the toughness and weldability of these steels that quite wide variations in the incidental constituents and the processing conditions can be tolerated without materially impairing these properties and a very wide range of strength and toughness can be achieved in what is basically a single class of steel. For example by finish rolling a steel containing 0.05% C, 1.9% Mn, 0.06% Nb, 0.011% N at 700 C., a yield strength of 41 tons/sq. in. can be obtained, coupled with a transition temperature of 85 C. and a Charpy shelf value of more than 220 ft. lb. On the other hand by normalising at 900 C. and air cooling a steel containing 0.05% C, 1.56% Mn, 0.27% Si, 0.05% Nb, 0.06% V,

4 0.013% N,.a yield strength of 24 tons/sq. in. can be obtained coupled with a transition temperature of -130 C. and a Charpy shelf value of more than 220 ft. lb.

The removal of pearlite also reduces the extent of segregation of the various constituents in the steel and in particular, reduces the incidence of laminations which in other steels are most often caused by the presence of pearlite.

Production and testing of many alloys within the scope of the present invention has enabled us to determine that the alloys according to the invention all have a yield strength of more than 22 tons/sq. in., a Charpy impact transition temperature at ft. lbs. of less than 0 C., and a Charpy shelf impact absorbed energy value of more than 100 ft. lbs. This combination of properties is superior to that possessed by known structural steels which meet British standard specifications 548, 1934; 968, 1941; and 968, 1962.

The following examples are given by way of illustration only:

Example 1 A steel of the following composition, the balance being iron, was prepared:

0.06% C; 0.022% S; 0.04% P; 1.32% Mn; 0.034% Cr; 0.0272% Ni; 0.008% Sn; 0.0022% N; 0.032% Nb.

The yield strength of the steel was in the range 27-30 tons/sq. in., its Charpy transition temperature was less than -60 C. and its weldability was much superior to any other structural steel presently available.

Examples 2-19 Steels of the compositions indicated below were prepared and finish rolled at the temperatures indicated and the lower yield strength (in tons/sq. in.), Charpy impact transition temperature (in C. at 40 ft. lbs.) and Charpy impact shelf energy (in ft. lbs.) of the finished steels were determined. In each case, the balance of the alloy composition was iron and unavoidable impurities.

Yield Impact rolling strength, Trans. shelf temp, tons/sq. temp, energy,

Ex. 0 o 0 Mn Nb N0 v 51 in. e c ft. lbs

940 28. 5 -38 214 2 835 04 1. 51 0.13 .054 .23 1 202 1, 019 28. -27 100 5 035 .041 1. 25 .02 .049 .24 23. 95 -4s 209 830 29. -72 212 1,020 '30. -2 174 4 955 .041 1. 97 125 020 .115 31. 0 -74 210 855 32. 1 -1l8 216 5 .035 1.55 .07 .023 .13 3%; g% D 30. 7 -55 185 0, s55 052 1. 59 .022 .12 1.11 34. 85 210 735 40. 3 --75 107 1, 020 33. 75 -14 105 7 040 .045 1. 52 .025 12 .20 33. 1 -05 122 835 33. 5 -12O 216 905 23. 05 -74 213 8 350 .04 1. 7 .135 .017 .25 30. 35 -105 213 770 31. 25 -108 116 1, 010 28. 55 -19 138 0 040 2. 03 135 0145 1 53 765 31. 0 216 10 044 2. 03 13 .024 25 3 3; 1, 020 28. 1 0 11- 940 .035 1. 41 0. 75 00s 20. a -02 200 830 29. 4 -112 205 1, 040 22. 65 -21 12 050 .0305 1. 49 .033 007 24. 0 -55 200 5 1 21 13 055 1.23 .054 .005 2M5 42 215 950 28. 55 -21 216 14 725 040 1. 91 .051 011 31. s -54 215 690 41. 0 -85 211 915 I 29. 95 -48 204 15 345 .049 1.81 .00 .0145 30. 7 -105 215 g5 -03 215 5 -3 210 10 860 01s .25 .053 .003 M 7 216 950 26. 1 -50 215 17 s00 .04 1. 52 .052 .0055 23. 3 -103 210 700 36. 25 -65 216 940 29. 4 -74 164 1s 340 039 2. 13 .097 .013 .93 30. 0 -102 204 735 35. 05 -103 1,015 31. 45 -28 205 10 035 .03 1. 55 104 010 29. 85 -77 215 770 29. 2 -87 21G In the case of the product of Example 4, finish rolled at 1020 C., the figure with asterisk in the Yield Strength column is the value of Proof Stress, in tons/sq. in., for an 0.2% permanent set.

The alloy compositions of Examples 2 to 19 all contained 2% by volume or less of pearlite.

I claim:

1. An alloy steel having a fine grained ferritic microstructure containing precipitated carbides or carbonitrides of at least one of the elements niobium and vanadium and not more than 2% by volume of pearlite, a yield strength minimum of 22 tons per square inch, a Charpy impact transition temperature at 40 ft. lbs. of less than 0 C., and a Charpy shelf impact absorbed energy value of more than 100 ft. lbs., said alloy steel consisting of up to 0.08% carbon, from 0.85 to 2.5% manganese, from 0.001 to 0.03% nitrogen, up to 0.5% silicon, and at least one of the elements niobium and vanadium in the proportions 0.01 to 0.20% Nb and 0.01 to 0.30% V, the balance being iron and incidental impurities and quantities of such elements which may be required to facilitate the steelmaking practice.

2. An alloy steel having a fine grained ferritic microstructure containing precipitated carbides or carbonitrides of at least one of the elements niobium and vanadium and not more than 2% by volume of pearlite, a yield strength minimum of 22 tons per square inch, a Charpy impact transition temperature at 40 ft. lbs. of less than 0 C., and a Charpy shelf impact absorbed energy value of more than 100 ft. lbs., said alloy steel consisting of from 0.01 to 0.04% carbon, from 1.4 to 2.4% manganese, from 0.001 to 0.03% nitrogen, up to 0.5% silicon, and at least one of the elements niobium and vanadium in the proportions 0.01 to 0.20% Nb and 0.01 to 0.3% V, the balance being iron and incidental impurities and quantities of such elements which may be required to facilitate the steelmaking practice.

3. An alloy steel according to claim 1 in which the contents of the at least one of niobium and vanadium is from 0.03 to 0.06%.

4. An alloy steel according to claim 1 in which the manganese content is from 1.25 to 2.00%.

5. An alloy steel having a fine grained ferritic microstructure containing precipitated carbides of carbonitrides of at least one of the elements niobium and vanadium and not more than 2% by volume of pearlite, a yield strength minimum of 22 tons per square inch, a Charpy impact transition temperature at 40 ft. lbs. of less than 0 C., and a Charpy shelf impact absorbed energy value of more than 100 ft. lbs., said alloy steel consisting of up to 0.60% carbon, from 0.85 to 2.5% manganese, from 0.001 to 0.03% nitrogen, up to 1.2% silicon, and at least one of the elements niobium and vanadium in the 6 proportions 0.01 to 0.20% Nb and 0.01 to 0.30 V, the balance being iron and incidental impurities and quantities of such elements which may be required to facilitate the steelmaking practice.

6. An alloy steel according to claim 5 in which the content of the at least one of niobium and vanadium is from 0.03 to 0.06%.

7. An alloy steel according to claim 1 in which the manganese content is from 1.25 to 2.00%.

8. An alloy steel having a fine grained ferritic microstructure containing precipitated carbides or carbonitrides of at least one of the elements niobium and vanadium and not more than 2% by volume of pearlite, a yield strength minimum of 22 tons per square inch, a Charpy impact transition temperature at 40 ft. lbs. of less than 0 C., and a Charpy shelf impact absorbed energy value of more than 100 ft. lbs., said alloy steel consisting of up to 0.10% carbon, from 0.85 to 2.5% manganese, from 0.001 to 0.03% nitrogen, up to 0.5% silicon, and at least one of the elements niobium and vanadium in the proportions 0.01 to 0.20% Nb and 0.01 to 0.30 V, the balance being iron and incidental impurities and quantities of such elements which may be required to facilitate the steelmaking practice.

9. An alloy according to claim 3 wherein the manganese content is from 1.25 to 2.00%.

10. An alloy steel according to claim 5 wherein the manganese content is from 1.25 to 2.00%.

References Cited UNITED STATES PATENTS 3,010,822 11/1961 Altenburger et al. -123 3,102,831 9/1963 Tisdale 75123 X 3,163,565 12/1964 Wada 148143 3,173,782 3/1965 Melloy et al. 75-123 3,180,726 4/1965 Nakamura 148-12 X OTHER REFERENCES Journal of The Iron and Steel Institute, vol. 201, April 1963, relied on pages 317, 318, 322 and 323.

Journal of The Iron and Steel Institute, vol. 201, November 1963, relied on pages 944, 945, 948, 951-953, 955-959.

Grange et al.: B, Ca, Cb and Zr in Iron and Steel, John Wiley & Sons, N.Y., 1957, relied on page 171.

The Making Shaping and Treating of Steel, 7th ed., 1957, page 364.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 75123 

